1. Field of the Invention
The present invention relates to a liquid crystal display, and, more particularly, to a semi-transmission type liquid crystal display which reflects incident light coming from outside to provide a display light source and transmits light from a light source at the back.
2. Description of the Related Art
There is a reflection type liquid crystal display (LCD) known which has a reflector inside and reflects incident external light by the reflector to provide a display light source, thereby eliminating the need for a backlight as a light source and a transmission type liquid crystal display equipped with a backlight as a light source.
As the reflection type liquid crystal display can be designed with lower power consumption, thinner and lighter than the transmission type liquid crystal display, it is mainly used for a portable terminal. This is because as light input from outside is reflected at the reflector in the display, the light can be used as a display light source, thus eliminating the need for a backlight. The transmission type liquid crystal display has such a characteristic as having a better visibility than the reflection type liquid crystal display in case where ambient light is dark.
The basic structure of the existing liquid crystal displays comprises a liquid crystal of an TN (Twisted Nematic) type, a single sheet polarizer type, an STN (Super Twisted Nematic) type, a GH (Guest-Host) type, a PDLC (Polymer Dispersed Liquid Crystal) type, a cholesteric type or the like, a switching element which drives the liquid crystal and a reflector or backlight provided inside or outside a liquid crystal cell. Those ordinary liquid crystal displays employ an active matrix drive system which can achieve high definition and high image quality using thin film transistors (TFTs) or metal/insulating film/metal structure diodes (MIMs) as switching elements, and are equipped with a reflector or backlight.
As a liquid crystal display which has advantages of both the conventional reflection type liquid crystal display and transmission type liquid crystal display, a semi-transmission type liquid crystal display is disclosed (see Japanese Patent No. 2955277) which, as shown in FIG. 1, has gate interconnections 2 and source interconnections 3 so provided as to run around pixel electrodes 1 of an active matrix substrate and intersect each other perpendicularly, has thin film transistors 4 provided on the pixel electrodes 1, has the gate interconnections 2 and source interconnections 3 connected to the gate electrodes and source electrodes of the thin film transistors 4 and has reflection areas 5 of a metal film and transparent areas 6 of ITO formed in the pixel electrodes 1.
As the reflection areas and transparent areas are provided in the pixel electrodes, the backlight can be turned off when the ambient light is bright so that the liquid crystal display can be used as a reflection type liquid crystal display, and thus demonstrates lower power consumption that is the characteristic of the reflection type liquid crystal display. When the ambient light is dark, the backlight is turned on so that the liquid crystal display is used as a transmission type liquid crystal display, and thus demonstrates an improved visibility in case where ambient light is dark, which is the characteristic of the transmission type liquid crystal display. Hereunder, a liquid crystal display which can be used as a reflection type liquid crystal display and as a transmission type liquid crystal display will be called as a semi-transmission type liquid crystal display.
According to the conventional semi-transmission type liquid crystal display, however, incident light travels through the liquid crystal layer back and forth in the reflection area 5 and passes the liquid crystal layer in the transparent area 6, thus producing a difference in light path in the liquid crystal layer. This results in a retardation difference between both areas, which disables the maximization of the intensity of the output light. To solve the problem, the liquid crystal display described in Japanese Patent No. 2955277 has an insulating layer 8 provided under an transparent electrode 7 in the reflection area 5 and a reflector 9 arranged over or under the insulating layer 8, as illustrated in a cross-sectional view of a liquid crystal display shown in FIG. 2, thereby providing a difference between the thickness, dr, of the liquid crystal layer in the reflection area 5 and the thickness, df, of the liquid crystal layer in the transparent area 6.
FIG. 5 is a graph showing the results of computing the intensity, Ip, of the output light in transmission mode and the intensity, Iλ, of the output light in reflection mode. It is apparent that the intensities of the output light in transmission mode and in reflection mode differ depending on the thickness of the liquid crystal layer. The difference in light path between the reflection area 5 and the transparent area 6 is canceled to approximate the characteristic of the output light by setting the ratio of the thickness dr of the liquid crystal layer in the reflection area to the thickness dr of the liquid crystal layer in the transparent area to about 1:2. Because the thickness of the insulating layer 8 is about a half the thickness of the liquid crystal layer and should be several micrometers, the number of the fabrication processes is increased, thus impairing the planarization of the transparent electrode 7. An alignment film which is formed on the transparent electrode 7 in order to align the liquid crystal molecules is affected by the planarization of the transparent electrode 7. This brings about a problem of making effective alignment difficult in a rubbing process.
Further, as shown in FIG. 3, a step between the reflection area 5 and the transparent area 6 disturbs an electric line of force 10 produced between a lower substrate 11 and an opposite substrate 12, thus deteriorating the characteristics of the liquid crystal display. Furthermore, as shown in FIG. 4, in a liquid crystal layer 13 around the step portion between the reflection area 5 and transparent area 6 on the lower substrate 11, the relationship between the direction of alignment of the liquid crystal molecules and the pretilt angle of the liquid crystal molecules in the vicinity of the surface of the lower substrate 11 generates disturbance in the rotational direction of the liquid crystal molecules (reverse tilt disclination) at the time the liquid crystal display is operated, thus deteriorating the characteristics of the liquid crystal display.